The use of caps, particularly on pick-up trucks, has become very popular. A cap on a pick-up truck permits use of the cargo space during inclement weather and is generally a desirable and useful addition. Such caps, which are usually fabricated into a single unit from plastic and metal, are expensive to purchase, heavy in weight causing difficulty in installation and, when not in use require substantial storage space. Notwithstanding the desirability of a cap, under certain circumstances, such as a load having a large volume, it becomes necessary to remove the cap. A cap of the single unit design creates a problem of both removal and storage. In addition a single-unit cap is expensive to produce and to ship resulting in a comparatively high cost. In accordance with this invention, a cap is provided for use on a pick-up truck or similar type of open structure that can be purchased at a comparatively low cost in a kit form packaged in a box that is readily shipable at low cost and can be carried by an individual under his or her arm and, in addition thereto, can be installed in a very short time and repackaged back into its original container or a similarly small container in a similarly short time.